Wellbores are typically completed with a cemented casing across the formation of interest to assure borehole integrity and allow selective injection into and/or production of fluids from specific intervals within the formation. It is necessary to perforate this casing across the interval(s) of interest to permit the ingress or egress of fluids. Several methods are applied to perforate the casing, including mechanical cutting, hydro-jetting, bullet guns and shaped charges. The preferred solution in most cases is shaped charge perforation because a large number of holes can be created simultaneously, at relatively low cost. Furthermore, the depth of penetration into the formation is sufficient to bypass near-wellbore permeability reduction caused by the invasion of incompatible fluids during drilling and completion.
FIG. 1 illustrates a perforating gun 10 consisting of a cylindrical charge carrier 14 with explosive charges 16 (also known as perforators) introduced into the well casing on a cable, wireline, coiled tubing or assembly of jointed pipes 20. Any technique known in the art may be used to deploy the carrier 14 into the well casing. At the well site, the explosive charges 16 are placed into the charge carrier 14, and the charge carrier 14 is then lowered into the oil and gas well casing to the depth of a hydrocarbon bearing formation 12. The explosive charges 16 fire outward from the charge carrier 14 and puncture holes in the wall of the casing and the hydrocarbon bearing formation 12. As best depicted in FIG. 2, the tunnels created through the casing wall and into the formation 12 are relatively narrow. As the charge jet penetrates the rock formation 12 it decelerates until eventually the jet tip velocity falls below the critical velocity required for it to continue penetrating. Particulate debris 22 created during perforation leads to plugged tunnel tips 18 that obstruct the production of oil and gas from the well.
Perforation using shaped explosive charges is inevitably a violent event, resulting in plastic deformation of the penetrated rock, grain fracturing, and the compaction of particulate debris (casing material, cement, rock fragments, shaped charge fragments) into the pore throats of rock surrounding the tunnel. Thus, while perforating guns do enable fluid production from hydrocarbon bearing formations, the effectiveness of traditional perforating guns is limited by the fact that the firing of a perforating gun leaves debris 22 inside the perforation tunnel and the wall of the tunnel. Moreover, the compaction of particulate debris into the surrounding pore throats results in a zone 26 of reduced permeability (disturbed rock) around the perforation tunnel commonly known as the “crushed zone.” The crushed zone 26, though only about one quarter inch thick around the tunnel, detrimentally affects the inflow and/or outflow potential of the tunnel (commonly known as a “skin” effect.) Plastic deformation of the rock also results in a semi-permanent zone of increased stress 28 around the tunnel, known as a “stress cage”, which further impairs fracture initiation from the tunnel. The compacted mass of debris left at the tip of the tunnel is typically very hard and almost impermeable, further reducing the inflow and/or outflow potential of the tunnel and the effective tunnel depth (also known as clear tunnel depth).
The distance a perforated tunnel extends into the surrounding formation, commonly referred to as total penetration, is a function of the explosive weight of the shaped charge; the size, weight, and grade of the casing; the prevailing formation strength; and the effective stress acting on the formation at the time of perforating. Effective penetration is the fraction of the total penetration that contributes to the inflow or outflow of fluids. This is determined by the amount of compacted debris left in the tunnel after the perforating event is completed. The effective penetration may vary significantly from perforation to perforation. Currently, there is no means of measuring it in the borehole. Darcy's law relates fluid flow through a porous medium to permeability and other variables, and is represented by the equation seen below.
  q  =            2      ⁢      π      ⁢                          ⁢              kh        ⁡                  (                                    p              e                        -                          p              w                                )                            μ      ⁡              [                              ln            ⁡                          (                                                r                  e                                                  r                  w                                            )                                +          S                ]            Where: q=flowrate, k=permeability, h=reservoir height, pe=pressure at the reservoir boundary, pw=pressure at the wellbore, t=fluid viscosity, re=radius of the reservoir boundary, rw=radius of the wellbore, and S=skin factor.The effective penetration determines the effective wellbore radius, rw, an important term in the Darcy equation for radial inflow. This becomes even more significant when near-wellbore formation damage has occurred during the drilling and completion process, for example, resulting from mud filtrate invasion. If the effective penetration is less than the depth of the invasion, fluid flow can be seriously impaired.
To minimize perforating damage and optimize production of a tunnel, current procedures to clear debris from tunnels rely on applying a relatively large pressure differential between the formation and the wellbore, or underbalance, wherein the formation pressure is greater than the wellbore pressure. These methods attempt to enhance tunnel cleanout by controlling the static and dynamic pressure behavior within the wellbore prior to, during and immediately following the perforating event so that a pressure gradient is maintained from the formation toward the wellbore, inducing tensile failure of the damaged rock around the tunnel and a surge of flow to transport debris from the perforation tunnels into the wellbore. FIG. 3 depicts the cleaning surge flow in an underbalanced situation after explosive charges 16 are fired. As the fluid flows through the tunnels and egresses through the tunnel openings 24, it takes with it the debris 22 formed as a result of perforation. However, if the reservoir pressure and/or formation permeability is low, or the wellbore pressure cannot be lowered substantially, there may be insufficient driving force to remove the debris.
Thus, in a number of situations, it is difficult or even impossible to create a sufficient pressure gradient between the formation and the wellbore. For example, in heterogeneous formations—where rock properties such as hardness and permeability vary significantly within the perforation interval—and in formations of high-strength, high effective stress and/or low natural permeability, underbalanced techniques become increasingly less effective. Since all the tunnels are being cleaned up in parallel by a common pressure sink, perforations shot into zones of relatively higher permeability will preferentially flow and clean up, eliminating the pressure gradient before perforations shot into poorer rock are able to flow. Since the maximum pressure gradient is limited by the difference between the reservoir pressure and the minimum hydrostatic pressure that can be achieved in the wellbore, perforations shot into low permeability rock may never experience sufficient surge flow to clean up. In such circumstances the perforation efficiency may be as low as 10% of the total holes perforated.
To solve these problems, methods have been developed for creating a dynamic underbalance around the gun immediately after creating perforated tunnels. For, example, U.S. Pat. No. 7,121,340 discloses a pressure reducer positioned adjacent to a perforating gun for reducing post-detonation pressure within the gun to enhance the dynamic underbalance effect within the gun and cause well-bore fluid to flow into the gun. U.S. Pat. No. 6,732,298 uses a porous solid around a perforation gun, which is crushed when the gun is detonated to produce a new volume into which wellbore fluids can flow, thereby enhancing the transient pressure around the gun. Others take advantage of the volume within the gun to create a dynamic underbalance. However, this generally calls for a reduction in the number of shaped charges within the gun and therefore, a reduction in shot density and an increased risk of low perforation efficiency. Low perforation efficiency, inadequately cleaned tunnels and/or insufficient shot density limits the overall inflow and/or outflow potential of the well and the area through which fluids can flow, causing increased pressure drop and erosion and impairing fracture initiation and propagation. Consequently, there is a need for a method of creating dynamic underbalance while ensuring that substantially every charge effectively produces and substantially clears a tunnel.